For example, when reproducing a multilayered optical disc, the signal light intensity decreases, and there is a high possibility that an error occurs in signal reading. In order to solve this problem, a homodyne detection method of amplifying a detection signal by using light interference is known (see Patent Document 1).
In Patent Document 1, as a homodyne system for detecting a light beam in which a signal light beam and a reference light beam are made to interfere with each other, it is necessary to perform detection on sets of four signal light beams and reference light beams whose phase differences are different by 90 degrees, respectively. Specifically, detection is performed on the sets of signal light beams and reference light beams whose phase differences are set to 0 degrees, 90 degrees, 180 degrees, and 270 degrees, respectively. Each of these detections is performed by detecting the light intensity of the light beam in which the signal light beam and the reference light beam are made to interfere with each other.
Further, Patent Document 2 describes signal processing for correctly reproducing a signal even when a light receiving element and an amplifier are AC-coupled. Further, Patent Document 3 describes a reproducing apparatus that applies a homodyne system to an optical disk in which signals are each recorded on a land and a groove.
In the homodyne system, the component of the signal light beam amplified according to the light intensity of the reference light beam can be obtained as a reproduction signal. By amplifying the signal light beam in this way, a signal to noise ratio (SNR) of the reproduction signal can be improved. Furthermore, the obtained reproduction signal is not influenced by the phase difference between the signal light beam and the reference light beam, so that an optical path length difference adjustment (so-called optical path length servo) can be made unnecessary.